On Tue, Jun 26, 2007 at 09:12:45AM +0200, Andi Kleen wrote: > There are some potential performance benefits too: > - When you have a device that cannot address the complete address range > an IOMMU can remap its memory instead of bounce buffering. Remapping > is likely cheaper than copying.
But those devices aren't likely to be found on modern systems. > - The IOMMU can merge sg lists into a single virtual block. This could > potentially speed up SG IO when the device is slow walking SG lists. > [I long ago benchmarked 5% on some block benchmark with an old > MPT Fusion; but it probably depends a lot on the HBA] But most devices are SG-capable. > And you get better driver debugging because unexpected memory > accesses from the devices will cause an trapable event. That and direct-access for KVM the big ones, IMHO, and definitely justify merging. > > Does it slow anything down? > > It adds more overhead to each IO so yes. How much? we have numbers (to be presented at OLS later this week) that show that on bare-metal an IOMMU can cost as much as 15%-30% more CPU utilization for an IO intensive workload (netperf). It will be interesting to see comparable numbers for VT-d. Cheers, Muli - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
